Sea-Tac International Airport passengers will soon begin undergoing full-body scans as the federal government installs equipment that will help identify terrorist threats but poses concerns about privacy, health risks and longer waits in security lines.

The first advanced-imaging technology (AIT) units should be installed at security checkpoints in mid- to late September, said Dwayne Baird of the Transportation Security Administration, as part of a nationwide rollout of the technology already used in 48 U.S. airports, including Spokane and Boise, Idaho.

The scans, which effectively allow agents to see through clothes by scattering low-dose X-rays at a passenger's front and back, produce a blurry nude image that can be screened for nonmetallic items such as weapons and explosives hidden under clothes.

To quell privacy concerns, TSA is making the screening optional, has agreed not to store the images, and has set up a system so the pictures are viewed by a screener in another location where passengers can't be seen in person.

"Every passenger has the option to refuse to go through these," and walk through a metal detector instead, Baird said. Those who do will be subject to a pat-down, a procedure that takes extra time, but one that privacy experts recommend for those who feel uncomfortable.

"People should not just accept this as a foregone conclusion," said Ginger McCall, staff counsel for the Electronic Privacy Information Center in Washington, D.C. Her organization has filed sued to stop the use of body scans, charging they are the equivalent of a digital strip-search.

That group and others also cite heath concerns over radiation exposure, "and there are substantial concerns about effectiveness," said McCall, when it comes to the kinds of threats the machines will be able to detect.

The Department of Homeland Security ramped up installation of full-body scanners after a failed Christmas Day attack by Umar Farouk Abdulmutallab, the so-called "underwear bomber," who hid powdered explosives in his pants while on board a Northwest Airlines flight bound for Detroit.

But a report by the Government Accountability Office said the scanners might not have detected the bomb, and critics say they are not designed to find objects hidden in body cavities.

The TSA, however, says the scanners have been effective in finding potentially threatening objects, and attests to their safety, saying the radiation exposure is no more than a passenger would experience flying for two minutes at 30,000 feet.

Safety questions

Security officials point to positive evaluations by the Food and Drug Administration, the National Institute for Standards and Technology and the Johns Hopkins University Applied Physics Laboratory.

Scientists at the University of California and New York's Columbia University, however, have raised red flags and called for more study.

"The majority of their energy is delivered to the skin and the underlying tissue," the California scientists wrote in what they titled a "Letter of Concern" in April.

While the dose of radiation would be safe if it were distributed throughout the volume of the entire body, the dose to the skin may be dangerously high, they said.

"There are concerns among frequent fliers and airline crews who may be going through these scanners several times a day as they travel around the country," said Steve Lott, U.S. spokesman for the Geneva-based International Air Transport Association, whose members include 230 U.S. and foreign airlines.

His group fears the TSA is moving too fast, with not enough long-term planning to predict problems.

"There's a concern about who's keeping up these machines," said McCall. "Who's going to be checking them to make sure they're not emitting more radiation than anticipated?"

Kate Hanni, head of FlyersRights.org, a passenger-rights organization, says her group is particularly worried about children flying alone.

The California scientists said the risk of radiation emission to children and adolescents did not appear to have been fully evaluated.

"We have reports of small children traveling unaccompanied who have been forced to be scanned and have not been given an option [of pat-downs]," Hanni said.

Calls for slowdown

Citing the unresolved privacy and health issues, the European Commission recently moved to slow the rollout of similar technology in Europe, Lott said.

He said airlines would like the TSA do the same. That's not likely, however, given the government's goal to install 450 machines this year and an additional 500 units by the end of 2011.

After seeing TSA make progress in reducing wait times at security checkpoints, Lott said airlines are worried the trend could be reversed.

"We have very serious concerns that this will lead to longer lines at security, and we'll see a return to the hassle factor."

Baird said he doesn't expect that to happen, although those opting for a pat-down may have to wait until an agent is available. So far, he said, in airports where the body scanners have been installed, less than 2 percent of passengers have opted for pat-downs. They are done by a TSA worker who is the same sex as the passenger.

The scan takes 7 to 15 seconds. Passengers are asked to enter the unit, stand with their feet apart and their arms raised, then wait while the image is screened.

Passengers still will have to remove their shoes as well as items they might have in their pockets, such as handkerchiefs, that would not have set off a metal detector.

"There was a learning curve on the part of passengers and TSA personnel," said Terry Sharpe, operations manager at El Paso International Airport, where the TSA installed scanners in June.

"People said to themselves, 'My watch never set off the alarm, or I never had to remove my pen or my belt,' but now they do because TSA is not just testing for metals. They're looking for objects."